FIG. 1 is a schematic functional block diagram illustrating the architecture of a conventional TV tuner. As shown in FIG. 1, the TV tuner 100 comprises a low noise amplifier (LNA) 101, a tracking filter 103, a band-pass filter 105, a quadrature generator 107, a RF clock synthesizer 111, a phase shifter 113, a double quadrature mixer 115, a polyphase filter unit 117, an amplifying unit 119 and a channel select filter 121.
After a RF signal (or a wideband TV signal) is received by the low noise amplifier 101, the RF signal is amplified to a certain level. In addition, the gain value of the low noise amplifier 101 is controlled by an automatic gain control (AGC) loop. The output signal from the low noise amplifier 101 is transmitted to the quadrature generator 107 through the tracking filter 103 and the band-pass filter 105. According to the output signal from the band-pass filter 105, the quadrature generator 107 generates a set of quadrature signal I0 and Q0.
The quadrature generator 107 is for example a multi-stage passive polyphase filter. By switching different passive components, the center frequency of the passive polyphase filter may be adjusted in order to comply with different frequency bands. Consequently, by the quadrature generator 107, the output signal from the band-pass filter 105 may be converted into an in-phase signal I0 and a quadrature-phase signal Q0.
In response to a frequency-selecting signal, the RF clock synthesizer 111 generates a local oscillation signal LO according to the preset type of the TV tuner 100. The frequency-selecting signal is generated according to the channel selected by the user. For example, in a case that the TV tuner 100 is set as a digital TV tuner, the system is switched to a zero intermediate frequency configuration, and the frequency of the local oscillation signal LO is set to be the frequency of the selected channel. Whereas, in a case that the TV tuner 100 is set as an analog TV tuner, the system is switched to a low intermediate frequency configuration, and the frequency of the local oscillation signal LO is set to be slightly higher than the frequency of the selected channel by several hundreds of thousand hertz (KHz).
Then, the phase shifter 113 shifts the phase of the local oscillation signal LO is by 90 degrees to generate two quadrature signals I1 and Q1. The signals I0 and Q0 outputted from the quadrature generator 107 and the signals I1 and Q1 outputted from the phase shifter 113 are mixed by the double quadrature mixer 115, and thus two base frequency (or low intermediate frequency) quadrature signals I2 and Q2 are produced. In a case that the TV tuner 100 is set as a digital TV tuner, the signals I2 and Q2 are base band frequency signals. Whereas, in a case that the TV tuner 100 is set as an analog TV tuner, the signals I2 and Q2 are low intermediate frequency signals.
If the signals I2 and Q2 are low intermediate frequency signals, the desired channel of the signals I2 and Q2 are allowed to pass through the polyphase filter unit 117 but the mirror channels of the signals I2 and Q2 are filtered off. Whereas, if the signals I2 and Q2 are base band frequency signals, the signals I2 and Q2 are allowed to directly pass through the polyphase filter unit 117. Consequently, the polyphase filter unit 117 generates the signals I3 and Q3. The signals I3 and Q3 are successively processed by the amplifying unit 119 and the channel select filter 121, so that desired signal I5 and Q5 are produced.
Generally, the tracking filter of the TV tuner should have high linearity and low noise figure in order to filter off undesired frequency band and noise of the RF signal and eliminate the adverse influence on the back-end circuit. FIG. 2A is a schematic diagram illustrating a tracking filter disclosed in U.S. Pat. No. 7,539,470. As shown in FIG. 2A, the LC parallel type filter 501 is a tracking filter. The LC parallel type filter 501 is connected with an amplifier 502 in series. The amplifier 502 comprises a first transistor Mn51, a second transistor Mn52 and a third transistor Mn53. The gate of the first transistor Mn51 is connected with a first bias voltage Bias1. The gate of the second transistor Mn52 is connected with a second bias voltage Bias2. The drain of the second transistor Mn52 is connected with the drain of the first transistor Mn51. The source of the second transistor Mn52 is connected with the source of the first transistor Mn51, and used as a signal input terminal In. The gate of the third transistor Mn53 is connected with an output bias voltage Bias_out. The drain of the third transistor Mn53 is connected with the LC parallel type filter 501, and used as a signal output terminal Out. The source of the third transistor Mn53 is connected to the drain of the first transistor Mn51. A current source Is5 is connected to the source of the first transistor Mn51 to provide a bias current.
However, the amplifier 502 fails to simultaneously acquire high linearity and low noise figure. Moreover, as the operating frequency changes, the input impedance Zth of the LC parallel type filter 501 changes. FIG. 2B is a schematic plot illustrating the relationship between the gain value and the operating frequency of the LC parallel type filter of FIG. 2A. As shown in FIG. 2B, it is found that the gain value of the LC parallel type filter 501 is increased as the operating frequency is increased. Therefore, an additional gain compensation circuit is required to assure that the gain value of the LC parallel type filter 501 is unchanged as the operating frequency is increased.
FIG. 3A is a schematic diagram illustrating another conventional tracking filter. As shown in FIG. 3A, the tracking filter comprises an inductor L and a capacitor C. The inductor L is interconnected between a voltage source Vdd and a signal output terminal Vout. The capacitor C is interconnected between a ground terminal and the signal output terminal Vout. The tracking filter is also connected with an amplifier in series. The amplifier comprises a first transistor T1, a second transistor T2 and a variable resistor Rdeg. The first transistor T1 has a base connected with a signal input terminal Vin, a collector connected to the voltage source Vdd, and an emitter connected with a first current source I1. The second transistor T2 has a base connected with the voltage source Vdd, a collector connected to the signal output terminal Vout, and an emitter connected with a second current source I2. The variable resistor Rdeg is connected between the two emitters of the first transistor T1 and the second transistor T2.
The capacitance value of the capacitor C may be adjusted by changing the operating frequency of the tracking filter. Assuming that the impedance of the tracking filter is Zin, the gain value of the amplifier is obtained by the following formula:
  Gain  =      Zin                  R        ⁢                                  ⁢        deg            +              2        gm            
FIG. 3B is a schematic plot illustrating the relationship between the gain value and the operating frequency of the tracking filter of FIG. 3A. In a case that the resistance value of the variable resistor Rdeg is unchanged, the gain value of the tracking filter is increased as the operating frequency is increased. In other words, the gain value may be compensated by changing the resistance value of the variable resistor Rdeg. For example, if the tracking filter is operated at a low operating frequency f1, the gain value of the tracking filter is increased as the resistance value of the variable resistor Rdeg is decreased. Whereas, if the tracking filter is operated at a high operating frequency f2, the gain value of the tracking filter is decreased as the resistance value of the variable resistor Rdeg is increased. In such way, the gain value does not change as the operating frequency changes.
However, in a case that the tracking filter is operated at the high operating frequency, since the resistance value of the variable resistor Rdeg is increased, the noise figure is increased. Whereas, in a case that the tracking filter is operated at the low operating frequency, since the resistance value of the variable resistor Rdeg is decreased, the linearity is impaired. Moreover, due to the parasitic effect of a printed circuit board (PCB), the Q factor of the tracking filter is deteriorated.
FIGS. 4A˜4D schematically illustrate two other conventional LC tracking filters. These LC tracking filters are disclosed in for example IEEE ISSCC Dig. Tech., pp. 208-209, 2007, “A Sip tuner with integrated LC tracking filter for both cable and terrestrial reception”. These LC tracking filters have good linearity. As shown in FIG. 4A, the LC tracking filter comprises a buffer 410, an impedance transformer Li and a resonator 415. The resonator 415 comprises an inductor Lp and a capacitor Cp. A signal input terminal Vin is connected with the buffer 410. The impedance transformer Li is interconnected between the output terminal of the buffer 410 and a signal output terminal Vout. The inductor Lp and the capacitor Cp are interconnected between the signal output terminal Vout and a ground terminal. Moreover, since the impedance value Zbuffer of the buffer 410 is low, the use of the impedance transformer Li with a high inductance value may increase the impedance value to Zth1.
FIG. 4B is a schematic plot illustrating the relationship between the gain value and the operating frequency of the LC tracking filter of FIG. 4A. As shown in FIG. 4B, the LC tracking filter has better rejection in high frequency. However, the uses of two inductors Li and Lp increase the fabricating cost of the LC tracking filter.
This LC tracking filter of FIG. 4C has better linearity. As shown in FIG. 4C, the LC tracking filter comprises a buffer 420, an impedance transformer Ci and a resonator 425. The resonator 425 comprises an inductor Lp and a capacitor Cp. A signal input terminal Vin is connected with the buffer 420. The impedance transformer Ci is interconnected between the output terminal of the buffer 420 and a signal output terminal Vout. The inductor Lp and the capacitor Cp are interconnected between the signal output terminal Vout and a ground terminal. Moreover, since the impedance value Zbuffer of the buffer 420 is low, the use of the impedance transformer Ci with a low capacitance value may increase the impedance value to Zth2.
FIG. 4D is a schematic plot illustrating the relationship between the gain value and the operating frequency of the LC tracking filter of FIG. 4C. As shown in FIG. 4D, the LC tracking filter has inferior rejection in high frequency. In addition, the LC tracking filter is readily suffered from harmonic interferences of the local oscillation signal LO.
Moreover, since the performance of the tracking filter is affected by many factors (e.g. temperature, process variation and parasitic effect of the printed circuit board), the tracking filter needs to be calibrated once the user wants to use the tracking filter. As known, it is time-consuming to calibrate the tracking filter. In addition, the calibration circuit is very complicated.